A dielectric device for high frequency application such as a filter has been conventionally formed as described below.
First, there is prepared a green sheet made of a dielectric material, e.g., a ceramic material. Electrode paste including mainly silver and an organic binder in an appropriate quantity is applied to a desired position on the sheet by, e.g., silk printing, followed by drying, thereby obtaining an inner electrode. A detailed description will be given below. As schematically shown in FIG. 3, a screen stencil mask 3 with an opening having a desired shape, i.e., a screen mesh 2 in this example formed thereon is registered at a desired position on a ceramic green sheet 1. Next, electrode paste 4 is applied onto the mask 3 and spread in a direction, e.g., indicated by an arrow in FIG. 3 by the use of a rubber squeezer 5. As a result, the electrode paste 3 is applied onto the green sheet 1 through the screen mesh 2, followed by drying, thus forming an inner electrode.
Subsequently, as schematically shown in FIG. 4, another ceramic green sheet 7 is laminated on the green sheet 1 having the inner electrode 6 printed at the upper surface thereof, followed by pressing so as to bring both the sheets into close contact with each other. Thereafter, a sintering treatment is performed by heating. The green sheet 7 may be provided with an electrode at the upper surface thereof in a manner similar to the sheet 1, or may be a dummy green sheet having no electrode thereon. A multi-layer dielectric device is obtained by combining the desired number of pairs of a green sheet having an electrode formed at the upper surface thereof and a dummy green sheet having no electrode thereon. Finally, a given outer electrode is disposed at a predetermined side surface, thus forming a dielectric device.
FIG. 5 is a schematic cross-sectional view showing the conventional high frequency dielectric device formed as described above, partly cut away in a thickness direction. As is obvious from FIG. 5, in the conventional high frequency dielectric device, the inner electrode 6 is flat, and side edges 8 of the electrode 6 are sharply tapered. This is because the inner electrode 6 is pressed in the thickness direction of the dielectric device by pressing, in particular, before sintering the device.
As is well known, in the case where a current flows in a conductor, the current inside the conductor becomes smaller due to a skin effect while the current tends to be concentrated at the surface of the conductor. Particularly, this tendency becomes more prominent at higher frequency. That is, in the case where a high frequency current flows in a conductor, the current is concentrated on the surface of the conductor due to the skin effect so as to reduce an effective cross-sectional area of the conductor, resulting in an apparent increase in electric resistance. This induces an ohmic loss of a current component.
Here, if a high frequency current flows in the high frequency dielectric device shown in FIG. 5, the current is concentrated on the surface of the inner electrode 6 due to the skin effect. Particularly, since in the conventional high frequency dielectric device shown in FIG. 5, the inner electrode 6 is flat and the side edges 8 are tapered sharply, as shown in FIG. 5, the skin effect acts strongly in a lateral direction (lengthwise in FIG. 5). Therefore, the current is concentrated on the edge 8 having a smaller cross-sectional area, thereby increasing current density, so as to induce an enormous ohmic loss of a high frequency component.
Such an ohmic loss of the high frequency component caused by the high current density is not preferable in view of the properties of the dielectric device for high frequency application. An influence of such a loss becomes more serious particularly in the case of a multi-layer dielectric device.